Reconciling geochemical and physical tracers of groundwater-stream interactions
Abstract
Calculations of groundwater inflows to streams assuming that groundwater and surface runoff are the two major reservoirs made using environmental tracers (e.g., Cl or Rn) and physical techniques based on analysing stream hydrographs commonly yield different results. In general, geochemical techniques produce lower estimates of baseflow than physical techniques and, even within the suite of geochemical tracers, conservative tracers such as Cl may produce different estimates of baseflow than tracers that have a time-dependence (e.g., Rn). While some of the discrepancies may relate to sampling issues, for example heterogeneous or poorly known groundwater geochemistry, the systematic differences indicate that the different techniques yield complementary information. The near-stream environment includes several transient water reservoirs, including the river banks, soils and the unsaturated zone, and surface water on floodplains all of which provide a delayed input of water to the stream over varying timescales that is geochemically distinct from groundwater. Thus, there are likely to be a variety of "slow flow" components all of which are geochemically distinct. In addition, sampling strategies differ. Hydrographs provide high temporal resolution but commonly are available for a single or few localities in a catchment; thus the different reservoirs contributing to the stream flow are averaged over large areas. By contrast, most geochemical campaigns involve sampling at high spatial resolution over a few sampling rounds. While this is suitable identifies reservoirs on a reach-scale, it yields little information as to how the reservoirs change with time. This study integrates work from several river catchments in SE Australia (Ovens, Barwon, Tambo, Avon, and Gellibrand). These catchments have well-constrained river flows and groundwater geochemistry, together with river and bank geochemistry (major ions, stable isotopes, and Rn). The combined data allow us to begin quantifying the input of water to the streams from reservoirs such as the river banks and the unsaturated zone and to understand the impacts of sampling prior to or following flood peaks on the stream geochemistry. These studies provide a more robust understanding of groundwater-surface water interaction in these catchments that goes to reconciling the results from different types of studies.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H41F1121H
- Keywords:
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- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 1804 HYDROLOGY / Catchment;
- 1830 HYDROLOGY / Groundwater/surface water interaction